The present invention relates to metal cutting inserts and to a method of making such inserts.
A prior art diamond-shaped cutting insert 8, which, for example, can be of the type used for metal turning operations is depicted in FIGS. 1 and 3. That insert comprises a sintered insert body 10 coated with a coating C such as an aluminum oxide coating. The insert 8 includes first and second oppositely situated main faces 12, 14 which are interconnected by side faces 16 as depicted in FIG. 3. The first main face 12 includes four cutting edges 18 and a chip breaking groove 20 associated with each cutting edge. A center hole 21 extends through the insert for receiving a fastener (not shown) to connect the insert to a seating surface (e.g., a shim surface) of a holder. The second main face 14 includes an annular supporting surface 22 which surrounds a central groove 23, the groove 23 surrounding the center hole 21. The supporting surface 22 is adapted to rest upon the seating surface of the holder. The supporting surface should seat in a flat and flush condition upon the seating surface in order to ensure that the cutting edge will not wobble during a cutting operation, and so that the area of surface contact is great enough to conduct heat from the insert at a sufficiently high rate to avoid damage to the insert.
A typical method of making such a metal cutting insert includes forming the insert body 10' by sintering a powderous hard material such as tungsten carbide. The sintering operation produces the insert body 10' having main faces 12', 14', side faces 16', cutting edges 18', chip breaking grooves 20', and a supporting surface disposed on the main face 14'. The supporting surface produced directly by the sintering step is not sufficiently smooth to achieve the optimum flat and flush seating of the insert on the holder seating surface discussed above. Therefore, it has been the practice to subject the sintered supporting surface to a grinding operation (prior to the coating step) in order to produce the smooth supporting surface 22'G depicted in FIG. 2. (Note: the suffix "G" as used herein denotes a surface which has been ground, whereas the suffix "UG" denotes an unground surface.)
Subsequent to the grinding of the supporting surface 22'G, the insert body 10' is coated with the coating C in order to impart desired conventional characteristics to the cutting edges 18 of the insert. Typically, the coating is applied by a conventional vapor deposition process such as a conventional chemical vapor deposition (CVD) process or a conventional physical vapor deposition (PVD) process or a combination of both.
Inserts made according to the above-described method have functioned adequately when used in non-precision cutting operations, but not as well in connection with precision cutting operations. The reason for this is that the vapor deposition process may produce so-called adhesions. Adhesions are small coating particles (surface bumps) which, when located on the supporting surface 22, may be large enough to disrupt the seating of the insert on the seating surface. That is, the insert may not seat flat and flush against the seating surface, whereby the insert may tend to wobble during the cutting operation, and also the area of surface contact is reduced. Insert wobbling will adversely affect the cutting accuracy, which is unacceptable for high precision cutting operations. The reduced surface area contact will slow the rate of heat conduction, thereby trapping more heat in the insert, whereupon an accelerated breakdown and/or wearing of the insert may occur.
It would, therefore, be desirable to provide methods for producing novel metal cutting inserts which avoid those shortcomings.